Process and apparatus for cryogenic air separation

ABSTRACT

The process and the apparatus in accordance with the invention relate to cryogenic separation of air in a distillation column system that has at least one single column ( 12 ). A compressed feed air stream ( 6, 8 ) is cooled in a main heat exchanger ( 9 ) in counter-current flow to a first return stream ( 16, 23 ) from the distillation column system. Cooled feed air stream ( 11 ) is fed into the distillation column system. A nitrogen-rich fraction ( 15 ) is produced in the upper region of the single column ( 12 ). At least part ( 16   b ) of the nitrogen-rich fraction ( 15 ) is condensed in a top condenser ( 13 ), which is constructed as a condenser-evaporator. At least part ( 54 ) of the liquid nitrogen-rich fraction ( 52 ) produced in the top condenser ( 13 ) is fed into the single column ( 12 ) as reflux. An oxygen-containing recycle fraction ( 18   a ) is drawn off from the single column ( 12 ) in liquid form. The liquid recycle fraction ( 18   a ) is cooled in a counter-current subcooler ( 100 ). The cooled recycle fraction ( 18   b ) is evaporated in the top condenser ( 13 ). The evaporated recycle fraction ( 29 ) is re-compressed in a re-compressor ( 30 ). The re-compressed recycle fraction ( 31, 32 ) is fed to the lower region of the single column ( 12 ). The main heat exchanger ( 9 ) and the counter-current subcooler ( 100 ) are formed as an integrated heat exchanger ( 102 ). The first return stream ( 16, 23 ) is fed into a group of passages ( 102 ) within the integrated heat exchanger which extend from the cold end thereof to the warm end thereof, and, in the process, the first return stream is brought into indirect heat exchange with both the liquid recycle fraction ( 18   a ) and the feed air stream ( 8 ).

SUMMARY OF THE INVENTION

The invention relates to a process for cryogenic air separation whichhas at least one single column, wherein:

-   -   a compressed feed air stream is cooled down in a main heat        exchanger in counter-current flow to a first return stream        (i.e., returned from the distillation column system to the main        heat exchanger),    -   the cooled feed air stream is fed into the single column,    -   a nitrogen-rich fraction is produced in the upper region of the        single column,    -   at least part of the nitrogen-rich fraction is condensed in a        top condenser, which is constructed as a condenser-evaporator,    -   at least part of the liquid nitrogen-rich fraction produced in        the top condenser is fed into the single column as reflux,    -   an oxygen-containing liquid recycle fraction is drawn off from        the single column in liquid form,    -   the liquid recycle fraction is cooled down in a counter-current        subcooler,    -   the cooled liquid recycle fraction is evaporated in the top        condenser,    -   the evaporated recycle fraction is re-compressed in a        re-compressor, and    -   the re-compressed recycle fraction is fed to the lower region of        the single column.

Similar processes with residual gas recycling are known from DE 2261234,U.S. Pat. No. 4,966,002, U.S. Pat. No. 5,363,657, U.S. Pat. No.5,528,906, U.S. Pat. No. 5,934,106, U.S. Pat. No. 5,611,218, U.S. Pat.No. 5,582,034, US 2004244417, DE 19909744 A1, DE 19919933 A1, DE19954593 A1, US 2007204652 A1, DE 102006027650 A1 and EP 1995537 A2. Inthis case, and also in U.S. Pat. No. 4,966,002 and U.S. Pat. No.5,582,034, a counter-current subcooler is used, in which the liquidoxygen-containing recycle fraction is “subcooled”, i.e. cooled below itsboiling point.

Here, “single column” is understood to mean a distillation column whichis operated in a uniform pressure range—which means here that thepressure difference between top and bottom of the column is basedexclusively on the pressure loss of the vapour rising in the column—andin which both the feed air is fed in as main feed fraction and also thenitrogen product is produced in the form of part of the nitrogen-richfraction accumulating in the upper region of the column. Double-columnor triple-column processes for nitrogen/oxygen separation are thereforenot covered. However, a pure oxygen column, which is connected to thesingle column and is operated as a pure stripping column, is not ruledout.

Processes and apparatuses for the cryogenic separation of air aredescribed in general terms in Hausen/Linde, Tieftemperaturtechnik, 2ndEdition 1985, Chapter 4 (pages 281 to 337).

An aspect of the invention is to provide a process of the type mentionedabove, as well as a corresponding apparatus, which are economicallyparticularly beneficial.

Thus, in accordance with the invention, there is provided a process ofthe type mentioned above wherein (references numerals refer to those ofthe FIGURE):

-   -   the main heat exchanger (9) and the counter-current subcooler        (100) are formed as an integrated heat exchanger (101),    -   the integrated heat exchanger (101) having a first group of        passages for the first return stream (16, 23), which extends        from the cold end of the integrated heat exchanger (101) to the        warm end of the integrated heat exchanger (101),    -   the first return stream (16, 23) is introduced into this group        of passages (102) at the cold end and flowing through the        integrated heat exchanger (101) as far as the warm end thereof,    -   the first return stream (16, 23) being brought into indirect        heat exchange with both the liquid recycle fraction (18 a) and        with the feed air stream (8) in the integrated heat exchanger        (101), and    -   the cooled feed air stream (11) being withdrawn from the        integrated heat exchanger (101) in completely gaseous form and        being fed into the single column (12) in completely gaseous        form.

Surprisingly, the use of an integrated heat exchanger, which combinesthe functions of a main heat exchanger and a counter-current subcooler,permits any pre-liquefaction of the air to be avoided. As a result, allof the air fed to the column is able to rise and participate in therectification. Thus, the separation effect becomes higher and, overall,the process according to the invention is therefore particularlybeneficial. The precise layout of the integrated heat exchanger dependson the boundary conditions of the individual case and must be definedfor each plant by using the usual calculation tools of the processengineer.

Besides this, the integration according to the invention simplifies thedesign considerably with respect to the pipework. Since thecounter-current subcooler is given a substantially larger cross sectionas a result of the integration in the main heat exchanger, the liquidstreams that flow in counter-current relation to the gas streams areoffered an optimum heating surface area. It is merely necessary for aheat exchanger to be supported and piped in the coldbox. The absolutenumber of headers of the two heat exchangers decreases. The gas streams(residual gas to the turbine, product nitrogen, residual gas from theturbine) from the top of the coldbox do not have to be led via two fixedpoints (counter-current subcooler and main heat exchanger). Expansionloops can be dispensed with; the integrated solution permits a pipe runwith minimized pipe stresses.

The integration of main heat exchanger and counter-current subcooler iscertainly known from air separation processes having two or more columnsfor nitrogen/oxygen separation. However, this measure has not previouslybeen applied to processes of the type mentioned above, since themanufacturing outlay for a particularly long integrated heat exchangerdid not appear to be justified in single column processes. Thesurprising effect of the avoidance of pre-liquefaction of the air waspreviously unknown.

In principle, any heat exchanger type can be used as an integrated heatexchanger in the process according to the invention, for example ahelically coiled heat exchanger or else a straight pipe exchanger.However, the use of a plate-type heat exchanger, in particular a brazedaluminium plate heat exchanger, is particularly beneficial. In thiscase, the integrated heat exchanger is formed by a single plate-typeheat exchanger block.

It is particularly cost-effective if the single column constitutes theonly distillation column of the distillation column system.

In order to generate refrigerating capacity, a further oxygen-containingfraction can be expanded, producing work. Thus, for example, the processcan further comprise:

-   -   withdrawing a further oxygen-containing fraction (14 a) in        liquid form from the single column (12),    -   this further oxygen-containing liquid fraction (14 a) is cooled        down in the integrated heat exchanger (101),    -   the cooled further oxygen-containing fraction is evaporated in        the top condenser (13),    -   the evaporated further oxygen-containing fraction (19) is warmed        in the integrated heat exchanger (101) in counter-current flow        to air, and    -   the warmed further oxygen-containing fraction is expanded in an        expansion machine (21) to produce work, and    -   the temperature of the liquid further oxygen-containing fraction        (14 a), as it is introduced into the integrated heat exchanger        (101), is higher than the temperature of the cooled feed air        stream (11) as it is withdrawn from the integrated heat        exchanger (101).

The integrated heat exchanger is also used for the subcooling of thefurther oxygen-containing fraction, in that the liquid furtheroxygen-containing fraction is cooled down in the counter-currentsubcooler before its evaporation. The integration according to theinvention makes it possible to introduce the further oxygen-containingliquid fraction into the heat exchanger above the temperature of the airremoval. The temperature difference is, for example, 0.2 to 5 K. Thiscontributes to the avoidance of the pre-liquefaction.

In addition, before being subjected to work-producing expansion, theevaporated further oxygen-containing fraction is warmed up bycounter-current heat exchange with air in the integrated heat exchanger.

The further oxygen-containing fraction can, for example, have the samecomposition as the recycle fraction. In this case, the two fractions canbe led in common lines and passages until after the top condenser.

Alternatively, the oxygen-containing recycle fraction is removed fromthe single column at an intermediate point which is located at least onetheoretical or practical plate above the point at which the furtheroxygen-containing fraction is removed. In this case, separate lines andseparate passages must be provided for the two fractions in the topcondenser and possibly in the counter-current subcooler.

Advantageously, the expansion machine is coupled mechanically to there-compressor. As a result, the mechanical energy obtained during thework-producing expansion is used for re-compression. This is preferablythe only energy source for the drive of the re-compressor.

It is beneficial if the re-compressor is constructed as a coldcompressor. Here, a “cold compressor” is understood to mean an apparatusin which the gas to be compressed is fed in at a temperature which liesconsiderably below the ambient temperature, in general below 250 K,preferably below 200 K.

It is also beneficial if, in the process according to the invention, there-compressed recycle fraction is cooled in the integrated heatexchanger before being introduced into the lower region of the singlecolumn, the re-compressed recycle fraction being drawn off from theintegrated heat exchanger in completely gaseous form and led into thesingle column in completely gaseous form. The recycle fraction istherefore also free of pre-liquefaction and participates in therectification in the single column completely as rising vapor.Therefore, the pre-liquefaction is avoided completely in both feedstreams to the single column, namely in the feed air and in the recyclefraction.

According to an apparatus aspect, the invention provides an apparatusfor cryogenic air separation in a distillation column system,comprising:

-   -   at least one single column (12),    -   a main heat exchanger (9) for cooling a compressed feed air        stream (6, 8) in counter-current flow to a first return stream        (16, 23) from the distillation column system,    -   means (such as conduits or piping) for introducing the cooled        feed air stream (11) into the single column (12),    -   means (such as conduits or piping) for removing a nitrogen-rich        fraction (15) from the upper region of the single column (12),    -   a top condenser for condensing at least part of the        nitrogen-rich fraction, the top condenser being constructed as a        condenser-evaporator,    -   means (such as conduits or piping) for introducing the liquid        nitrogen-rich fraction (52) produced in the top condenser (13)        into the single column (12) as reflux,    -   means (such as conduits or piping) for withdrawing an        oxygen-containing recycle fraction (18 a) from the single column        (12) in the liquid state,    -   a counter-current subcooler (100) for cooling down the liquid        recycle fraction (18 a),    -   means (such as conduits or piping) for introducing the cooled        recycle fraction (18 b) into the top condenser (13),    -   a re-compressor (30) for compressing the evaporated recycle        fraction (29) from the top condenser (13), the re-compressor        (30) being constructed, for example, as a cold compressor, and    -   means (such as conduits or piping) for introducing the        re-compressed recycle fraction (31, 32) into the lower region of        the single column (12),        wherein    -   the main heat exchanger (9) and the counter-current subcooler        (100) are formed as an integrated heat exchanger (101),    -   the integrated heat exchanger (101) having a first group of        passages (102) for the first return stream (16, 23), which        extends from the cold end to the warm end of the integrated heat        exchanger,    -   the cold end of the integrated heat exchanger (101) being        connected to means (such as conduits or piping) for introducing        the first return stream (16, 23) into the first group of        passages,    -   the warm end of the integrated heat exchanger (101) being        connected to means (such as conduits or piping) for withdrawing        the first return stream (16, 23) from the first group of        passages,    -   the integrated heat exchanger (101) being constructed in such a        way that, during operation, the first return stream (16, 23) is        brought into indirect heat exchange with both the liquid recycle        fraction (18 a) and the feed air stream (8), and    -   the passages in the integrated heat exchanger (101) are arranged        in such a way that, during operation, the cooled feed air stream        (11) is withdrawn from the integrated heat exchanger (101) in        completely gaseous form and is fed into the single column (12)        in completely gaseous form.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and further details, such as features and attendantadvantages, of the invention are explained in more detail below on thebasis of an exemplary embodiment which is diagrammatically depicted inthe drawing, and wherein:

FIG. 1 shows an embodiment of the device according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Atmospheric air 1 is taken in by an air compressor 3, via a filter 2,and compressed to an absolute pressure of 6 to 20 bar, preferably about9 bar. After flowing through a re-cooler 4 and a water separator 5, thecompressed air 6 is cleaned in a cleaning device 7. The cleaning device7 has a pair of containers which are filled with adsorption material,preferably a molecular sieve. The cleaned air 8 is cooled down tosomewhat above the dew point in a main heat exchanger 9 and finally ledinto a single column 12 as a completely gaseous feed air stream 11.

The operating pressure of the single column 12 (at the top) is 6 to 20bar, preferably about 9 bar. Its top condenser is cooled with anoxygen-containing recycle fraction 18 a, 18 b and a furtheroxygen-containing fraction 14 a, 14 b. The further oxygen-containingfraction 14 a is drawn off from the bottom of the single column 12, therecycle fraction 18 a from an intermediate point some practical ortheoretical plates further above the bottom of the single column 12.Before they are fed 14 b, 18 b into the top condenser 13, both fractions14 a, 18 a are cooled down in a counter-current subcooler 100. The mainheat exchanger 9 and counter-current subcooler 100, according to theinvention, are formed by an integrated heat exchanger 101, which isimplemented here as a single plate-type heat exchanger block. The heightdifference between the exit of the stream 14 a from the single column 12(more precisely the liquid level at the bottom of the column) and theentry into the integrated heat exchanger 101 should in principle bechosen such that the proportion of gas as a result of the expansion liesbelow 5% by volume. If, in a departure from this, the proportion of gasis higher than 5% by volume, a perforated plate is fitted in the headerover the entire region above the point of entry of the two-phase mixtureinto the passages. The pressure loss across the perforated plate ischosen such that the gas bubbles are distributed over all the passages.The two-phase mixture is then fed into the integrated heat exchanger(101), first transversely with respect to the other streams (possiblywith one or more deflections), in which the gas proportion is condensedcompletely, that is to say the adjacent passages are correspondinglycolder in every operating case. After the initial transverse flow, thefluid streams flows counter-current to the other streams.

The main product from the single column 12, gaseous nitrogen 15, 16 isdrawn off at the top and, as first return stream, is led through a groupof passages 102 which extend from the cold end to the warm end of theintegrated heat exchanger. In the process, the recycle stream (16) inthe region of the counter-current subcooler 100 comes into indirect heatexchange with the two oxygen-containing fractions 14 a, 18 a and then,in the region of the main heat exchanger 9, into indirect heat exchangewith the feed air stream 8. Via a line 17, it is finally drawn off atapproximately ambient temperature as a gaseous pressurized product(PGAN).

The remainder 16 b of the gaseous nitrogen 15 is condensed completely orsubstantially completely in the top condenser 13. Part 53 of thecondensate 52 from the top condenser 13 can be removed as liquidnitrogen product (PLIN); the remainder 54 is introduced into the top ofthe single column as reflux. Non-condensed constituents can be drawn offvia a purge line 90.

The recycle fraction 18 b is evaporated in the top condenser 13 under apressure of 2 to 9 bar, preferably about 4 bar, and flows in gaseousform via line 29 to a cold compressor 30, in which it is re-compressedapproximately to a pressure which is sufficient to feed it back into thesingle column. The re-compressed recycle fraction 31 is cooled down tocolumn temperature again in the counter-current subcooler 100 and fed tothe single column 12 at or near the bottom in completely gaseous formvia line 32.

The further oxygen-containing fraction 14 b is evaporated in the topcondenser 13 under a pressure of 2 to 9 bar, preferably about 4 bar, andflows in gaseous form via line 19 to the cold end of the integrated heatexchanger 101. There, in the region of the counter-current subcooler100, it comes into indirect heat exchange with the two liquidoxygen-containing fractions 14 a, 18 a and then, in the region of themain heat exchanger 9, into indirect heat exchange with the feed airstream 8. It is removed from the main heat exchanger 9 again (line 20)at an intermediate temperature and is expanded to about 300 mbar aboveatmospheric pressure, producing work, in an expansion machine 21 which,in the example, is constructed as a turbo-expander. The expansionmachine is coupled mechanically to the cold compressor 30 and a brakingdevice 22 which, in the exemplary embodiment, is formed by an oil-filledbrake. The expanded further fraction 23 is warmed up to about ambienttemperature in the integrated heat exchanger 101. The warm furtherfraction 24 is blown off into the atmosphere (line 25) and/or used inthe cleaning device 7 as regeneration gas 26, 27, possibly followingheating in the heating device 28.

As mentioned above, the further oxygen-containing fraction (14 a, 14 b)can, for example, have the same composition as the oxygen recyclefraction (18 a, 18 b). For example, these two fractions can be removedfrom column 12 as a single stream, for example the stream (14 a, 14 b),and introduced as a single stream into top condenser 13. Thereafter, thestreams are divided into two streams (19) and (29).

In the exemplary embodiment, the top condenser 13 is constructed as aforced-flow evaporator. Alternatively, a bath evaporator or falling filmevaporator can be used.

The entire disclosure[s] of all applications, patents and publications,cited herein and of corresponding German Application No. 10 20099014557.5, filed Mar. 24, 2009 are incorporated by reference herein.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A process for cryogenic air separation in a distillation columnsystem comprising at least one single column (12), said processcomprising: cooling a compressed feed air stream (6, 8) in a main heatexchanger (9) in counter-current to a first return stream (16, 23) fromthe distillation column system, introducing the cooled feed air stream(11) into said single column (12), removing a nitrogen-rich fraction(15) from the upper region of said single column (12), condensing atleast part (16 b) of the nitrogen-rich fraction (15) in a topcondenser-evaporator (13), introducing at least part (54) of thecondensed liquid nitrogen-rich fraction (52) from the topcondenser-evaporator (13) into said single column (12) as reflux,withdrawing an oxygen-containing recycle fraction (18 a) from saidsingle column (12) in liquid form, cooling the oxygen-containing recycleliquid fraction (18 a) in a counter-current subcooler (100), evaporatingthe cooled oxygen-containing recycle fraction (18 b) in the topcondenser-evaporator (13), re-compressing the evaporatedoxygen-containing recycle fraction (29) in a re-compressor (30), andintroducing the re-compressed recycle fraction (31, 32) into the lowerregion of said single column (12), wherein the main heat exchanger (9)and the counter-current subcooler (100) are formed as an integrated heatexchanger (101), the integrated heat exchanger (101) having a firstgroup of passages for said first return stream (16, 23), which extendfrom the cold end of said integrated heat exchanger (101) to the warmend of said integrated heat exchanger (101), said first return stream(16, 23) being introduced into said first group of passages (102) at thecold end of said integrated heat exchanger (101), and flowing throughsaid integrated heat exchanger (101) to the warm end said integratedheat exchanger (101) and, during passage through said integrated heatexchanger (101), said first return stream is brought into indirect heatexchange with both said liquid recycle fraction (18 a) and said the feedair stream (8), and the cooled feed air stream (11) is withdrawn fromsaid integrated heat exchanger (101) in completely gaseous form and isfed into said single column (12) in completely gaseous form.
 2. Aprocess according to claim 1, wherein said integrated heat exchanger isa single plate-type heat exchanger block.
 3. A process according toclaim 1, wherein said single column is the only distillation column ofsaid distillation column system.
 4. A process according to claim 1,further comprising withdrawing a further oxygen-containing fraction (14a) from said single column (12) in liquid form, cooling the furtheroxygen-containing liquid fraction (14 a) in said integrated heatexchanger (101), evaporating the cooled further oxygen-containing liquidfraction in the top condenser-evaporator (13), warming the evaporatedfurther oxygen-containing fraction (19) in said integrated heatexchanger (101) in counter-current flow to air, and expanding the warmedevaporated further oxygen-containing fraction in an expansion machine(21) to produce work, wherein the temperature of the furtheroxygen-containing liquid fraction (14 a) as introduced into saidintegrated heat exchanger (101) is higher than the temperature of thecooled feed air stream (11) withdrawn off from said integrated heatexchanger (101).
 5. A process according to claim 4, wherein, before thework-producing expansion, the warmed evaporated furtheroxygen-containing fraction is warmed up in counter-current flow to airin said integrated heat exchanger.
 6. A process according to claim 4,wherein said oxygen-containing recycle fraction is removed from saidsingle column at an intermediate point which is located at least onetheoretical or practical plate above the point at which said furtheroxygen-containing fraction is removed from said single column (12).
 7. Aprocess according to claim 4, wherein said expansion machine (21) iscoupled mechanically to said re-compressor (30).
 8. A process accordingto claim 1, wherein said re-compressor (30) is constructed as a coldcompressor.
 9. A process according to claim 1, wherein saidre-compressed recycle fraction (31) is cooled in said integrated heatexchanger (101) before being introduced into the lower region of saidsingle column (12), and said re-compressed recycle fraction (32) iswithdrawn from said integrated heat exchanger (101) in completelygaseous form and fed into said single column (12) in completely gaseousform.
 10. An apparatus for cryogenic air separation in a distillationcolumn system, comprising: at least one single column (12), a main heatexchanger (9) for cooling a compressed feed air stream (6, 8) incounter-current flow to a first return stream (16, 23) from thedistillation column system, means for introducing a cooled feed airstream (11) into said single column (12), means for removing anitrogen-rich fraction (15) from the upper region of said single column(12), a top condenser-evaporator for condensing at least part of thenitrogen-rich fraction, means for introducing condensed nitrogen-richfraction (52) from said top condenser-evaporator (13) into said singlecolumn (12) as reflux, means for withdrawing an oxygen-containing liquidrecycle fraction (18 a) from said single column (12), a counter-currentsubcooler (100) for cooling down liquid recycle fraction (18 a), meansfor introducing cooled recycle fraction (18 b) into said topcondenser-evaporator (13), a re-compressor (30) for compressingevaporated recycle fraction (29) from said top condenser-evaporator(13), and means for introducing re-compressed recycle fraction (31, 32)into the lower region of said single column (12), wherein said main heatexchanger (9) and said counter-current subcooler (100) are formed as anintegrated heat exchanger (101), said integrated heat exchanger (101)having a first group of passages (102) for the first return stream (16,23), which extends from the cold end of said integrated heat exchangerto the warm end of said integrated heat exchanger, the cold end of saidintegrated heat exchanger (101) being connected to means for introducingthe first return stream (16, 23) into said first group of passages, thewarm end of said integrated heat exchanger (101) being connected tomeans for withdrawing the first return stream (16, 23) from said firstgroup of passages, the integrated heat exchanger (101) being constructedso that, during operation, the first return stream (16, 23) is broughtinto indirect heat exchange with both liquid recycle fraction (18 a) andfeed air stream (8), and the passages in the integrated heat exchanger(101) are arranged so that, during operation, cooled feed air stream(11) is withdrawn from said integrated heat exchanger (101) incompletely gaseous form and is fed into said single column (12) incompletely gaseous form.